Turn the AC off
The ac compresser will run in the winter. The only modes which the AC does NOT run are: OFF, VENT, and FEET ONLY. (this is in the owners manual)
The reason the ac runs:any time that air is directed towards glass, the AC compressor is cycled to reduce the humidity of the air (cold air can't support as much water vapour as warm air) in order to minimize fogging. Through the marvels of modern engineering, the air can be passed through the heater core after it is passed through the evaporater core.
How this helps.
not sure if this is true for all years and models, but is for mine (98 5.4 eddie)



